Mercedes has discovered a turbo issue was behind Valtteri Bottas' retirement from the Spanish Grand Prix, a defect that Toto Wolff reveals the team has never encountered before.

Fresh from his maiden F1 win in Russia, Bottas' burgeoning title aspirations took a major knock when he was forced to retire whilst running third in Barcelona.

A rare Mercedes power unit mechanical issue, the team set to work on investigating the problem and have now diagnosed a turbo problem that Wolff admits has not been seen before.

"It was painful to lose 15 valuable points with Valtteri retiring from P3. We've identified the root cause of the problem, which was the turbo. We haven't seen that defect before, which shows you that you need to be double diligent. This is a technical sport and if you stretch your limits, you'll encounter technical problems."

Pointing out that the margins for error are becoming smaller as the pendulum of momentum swings between Mercedes and Ferrari, Wolff admits limits are being pushed.

"It's always good to come away from a race weekend with a points advantage, no matter how small. But the gap is just that: very small. We are in a massive fight with Ferrari. On one side, this is very exciting and challenging. But on the other side, it is going to stretch us to our limits. You cannot base your current assessment on the balance of power on just Barcelona."

Ahead of this weekend's Monaco Grand Prix, Mercedes leads Ferrari by eight points in the constructors' standings, though Sebastian Vettel retains a slim six point margin over Lewis Hamilton in the drivers' reckoning.

Bottas remains third in the standings, though his DNF has leaves him 41 points adrift of Vettel in the standings.